Velocity fluctuations are predominant force of the indoor-outdoor air-exchange in pumping ventilation (PV). However, few studies have given a quantitative evaluation on their exact contribution to the ventilation rate or pollutant removal in PV. This study investigates airflow and concentration fields in rooms of an isolated three-story building under PV with different opening separations (normalized opening-center distances between two openings) using Large-Eddy Simulation (LES). The purpose is to validate numerical accuracy against previous wind tunnel experiments (WTE) and elucidate mass transport mechanisms governing ventilation efficiency of PV. LES analyzed mean and fluctuating stream-wise velocities in the building wake, indoor tracer gas concentrations, and effective ventilation rates, comparing them quantitatively with WTE data. Convective and unsteady mass fluxes were quantified to delineate their respective contributions to pollutant removal. Quadrant analysis of unsteady velocities and concentration at the opening center have been conducted to describe the unsteady mass transport characteristics through the opening in PV. The results demonstrated that LES accurately predicted mean and fluctuating velocity statistics (mean deviation < 5%) and effective ventilation rates (within 10% error for center-point based purging flow rate). LES underestimated both the mean and fluctuating concentration with deviations generally below 20%. The unsteady mass flux accounted for over 90% of the total mass flux through the opening. Quadrant analysis indicated that the ejection and sweep were much more significant than the inward interaction and outward interaction. Larger opening separations could enhance unsteady mass flux and the ejection event at the opening.
Gui et al. (Wed,) studied this question.